1. Field of the Invention
The present invention relates to bi-directional communication systems and, in particular, to techniques for enhancing noise immunity of such communication systems.
2. Description of the Related Art
Bi-directional communication has several advantages over unidirectional communication. For example, in some applications, physical connection points may be reduced by a factor that approaches two. In a chip-to-chip communication application where pin out, bonding pad or other connection point limitations may be significant, bi-directional communication can allow wider data widths using a fixed number of connection points. In other applications, transmission, routing or switching complexity may be reduced by replacing separate unidirectional facilities.
In a bi-directional communication system, a pair of transceivers transmit and receive data signals via a bi-directional transmission line or channel. Signals simultaneously transmitted by the transceivers are superimposed as a combined data signal. This combined data signal typically includes system noise from each of the transceiver systems. For example, in an application where transceivers are implemented as part of separate integrated circuit chips with separate supply voltage distribution systems, each transceiver may experience source of supply voltage noise or disturbance, e.g., switching noise, voltage bounces and collapses. Such supply voltage noise manifests itself in signals communicated from chip to chip. For example, data signals exhibit differential and/or common-node noise.
To realize the benefits of bi-directional signaling, the signal sent into the transmission line by a local transmitter is removed from the combined signal to recover the received signal, i.e., to recover the signal transmitted by a remote transceiver. Echo cancellation techniques can be used to remove local transmitter contributions. However, technical challenges exist in performing echo cancellation with low phase and amplitude errors, without significant additional pin overhead, and without coupling crosstalk noise into adjacent signaling channels. Furthermore, it may be desirable to employ transmitter-side techniques such as pre-distortion to account for channel characteristics such as intersymbol interference (ISI) and thereby improve channel capacity. Predistortion presents additional technical challenges.
Mooney et al. disclose a bi-directional communications scheme that employs a pair of bi-directional references (corresponding to high and low voltages, respectively) and switches between the respective references based on the current signal voltage of outgoing transmitted data. See Mooney et al., A 900 Mb/s Bidirectional Signalling Scheme, IEEE Journal of Solid-State Circuits, Vol. 30, No. 12 (1995). Mooney""s technique has several limitations. First, since the bi-directional references are shared amongst multiple channels to reduce pin overhead, the references will experience data-dependent noise such as from switched capacitor loading associated with each channel""s data-dependent selection of the appropriate high or low reference channel. In addition, since Mooney""s references are respectively biased at high and low voltages, they do not track differential supply collapses and bounces. Furthermore, Mooney""s data dependent selection of the appropriate high or low reference is unable to account for variation, such predistortion, of a local transmitter output.
Ishibashi et al. disclose bi-directional transceiver logic for a crossbar switch. See, Ishibashi et al. SBTL (Simultaneous Bi-directional Transceiver Logic) for a 26.8 GB/s Crossbar Switch, Hot Interconnects VI Symposium Record (August 1998). Ishibashi employs a local echo signal to extract incoming data from a combined bi-directional signal. However, a local-only reference is used and no provision is made for tracking supply noise at an opposing end of the bi-directional transmission line. Accordingly, Ishibashi""s scheme is vulnerable to both common mode and differential supply noise at the opposing end.
Improved techniques are desired. For example, techniques are desired whereby both common-node and differential supply noise may be tracked. In particular, receiver techniques are desired whereby common-node and differential supply noise at both ends of a bi-directional transmission line can be tolerated. In addition, techniques are desired whereby references employed by a receiver may account for pre-distortion of an outgoing signal, if employed by the local transmitter. Furthermore, it is desirable that reference circuitry employed in a transceiver implementation avoid coupling data dependent switching noise between transmission channels. Some or all of the foregoing features and advantages are realized by various systems, circuits and methods in accordance with the present invention.
In one embodiment in accordance with the present invention, a bi-directional communication system includes a pair of integrated circuits, each implementing a transceiver for communication on a bi-directional transmission line, wherein an additional bi-directional reference line is utilized to compensate for both common-mode and differential supply noise introduced at either integrated circuit.
In another embodiment in accordance with the present invention, a transceiver for bi-directional communications via a channel includes a bi-directional data node for coupling to the bi-directional communications channel, a bi-directional reference node for coupling to a bi-directional reference channel, and a receiver circuit. The receiver circuit, which is coupled to both the bi-directional data node and the bi-directional reference node, tracks both common-mode and differential noise introduced at an opposing end of the communications channel. In one variation, the transceiver includes a transmit circuit that predistorts an outgoing signal transmitted via the bi-directional communications channel. The receiver circuit of such a transceiver combines a predistorted data-dependent local echo signal and a data-independent bi-directional reference signal coupled from the bi-directional reference node. In another variation, additional bi-directional data nodes are provided for coupling to respective additional bi-directional communication channels and the receiver circuit extracts an incoming signal from that presented at the bi-directional data node without reference channel switching induced crosstalk amongst the bi-directional communication channels.
In another embodiment in accordance with the present invention, an apparatus includes a transmit circuit coupled to a bi-directional data node, an echo data node, a local reference node, a bi-directional reference node and a receiver circuit. The receiver circuit is coupled to the bi-directional data node, the echo data node, the local reference node and the bi-directional reference node to combine signals presented thereon. The receiver circuit extracts an incoming signal component from that presented at the bi-directional data node and compensates for both common-mode and differential noise introduced at either end of a bi-directional communications channel coupled thereto.
In still another embodiment in accordance with the present invention, a bi-directional communication system includes a bi-directional reference channel, plural bi-directional communication channels, and a first transceiver including plural receivers each coupled to a respective one of the bi-directional communication channels. The receivers are further coupled to the bi-directional reference channel and to respective local reference nodes. In one variation, the bi-directional communication system includes a second transceiver including plural receivers each coupled to a respective one of the bi-directional communication channels. These receivers are further coupled to the bi-directional reference channel and to respective reference nodes local thereto. In some variations, the first and second transceivers reside on separate integrated circuit chips.
In still yet another embodiment in accordance with the present invention, bi-directional signaling method includes receiving on a bi-directional transmission line, a data signal that includes both an incoming signal and an outgoing signal transmitted thereon; receiving on a bi-directional reference transmission line, a bi-directional reference signal including both an incoming reference and an outgoing reference contribution thereto; and obtaining the incoming signal from the data signal using a composite data-dependent reference including contributions of the bi-directional reference signal, a local reference signal, and a local echo signal corresponding to the outgoing signal.